A Novel Reduced Graphene Oxide-Attapulgite (RGO-ATP) Supported Fe₂O₃ Catalyst for Heterogeneous Fenton-like Oxidation of Ciprofloxacin: Degradation Mechanism and Pathway

Round 1

Reviewer 1 Report

In this manuscript, the authors present a novel reduced graphene-attapulgite (rGO-ATP) supported Fe₂O₃ catalyst for heterogeneous Fenton-like oxidation of Ciprofloxacin: degradation mechanism and pathway. The topic is sufficient interest to warrant publication. However, some points need to be addressed before the publication.  

The manuscript is poorly organized content making it difficult to read. Authors should convince/ introduce why combining Fe₂O₃ with reduced graphene-attapulgite (rGO-ATP) is suitable/ effective for using in Ciprofloxacin degradation. Please reorganize and elaborate the highlight and more clearly the necessity of conducting this work.

Page 3: Some information should be provided such as specific surface area, total pore volume of the as-synthesized materials in order to complete the explanation.  

Page 7 line 220: In Figure 5(a)-(c), it is really difficult to compare the fluorescence characteristics of 3 images because there are shown the different color scales. In order to clarify and confirm the exact explanation, Figure 5(a)-(c) should be rescale and provide some more explanation.

Page 10 line 304: Please recheck equation (1) and definition of “The degradation rate of Ciprofloxacin”. and explain more what is C₀ and C_t   

Page 10 line 331: A more detailed description of characterization techniques is needed. (add more information about XPS)

Which component of your materials play an important role to degrade Ciprofloxacin?

A comparison table would be interesting to show earlier reported work in “heterogeneous Fenton-like oxidation of Ciprofloxacin” area.

Author Response

Dear reviewer

Thanks for your useful comments and suggestions on the language and the structure of our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point:

First:

The manuscript is poorly organized content making it difficult to read. Authors should convince/ introduce why combining Fe₂O₃ with reduced graphene-attapulgite (rGO-ATP) is suitable/ effective for using in ciprofloxacin degradation. Please reorganize and elaborate the highlight and more clearly the necessity of conducting this work.

--We have reorganized content to make it easy to understand. And we have explained combining Fe₂O₃ with reduced graphene-attapulgite (rGO-ATP) is suitable/ effective for using in Ciprofloxacin degradation in page 3 lines 110-115.

Page 3: Some information should be provided such as specific surface area, total pore volume of the as-synthesized materials in order to complete the explanation.

-- We have provided specific surface area, total pore volume of the as-synthesized materials in page 4 lines 142-146.

Page 7 line 220: In Figure 5(a)-(c), it is really difficult to compare the fluorescence characteristics of 3 images because there are shown the different color scales. In order to clarify and confirm the exact explanation, Figure 5(a)-(c) should be rescale and provide some more explanation.

We have corrected the explanations of Figure 5 in pages 10.

Page 10 line 304: Please recheck equation (1) and definition of “The degradation rate of Ciprofloxacin”. and explain more what is C₀ and Ct.

-- We have rechecked equation (1) and definition of “The degradation rate of Ciprofloxacin” and explain more what is C₀ and C_t in page 14.

Page 10 line 331: A more detailed description of characterization techniques is needed. (add more information about XPS)

-- We have given more information about XPS in page 14.

Which component of your materials play an important role to degrade Ciprofloxacin?

-- The active component is Fe₂O₃ loaded on RGO-ATP, which can catalyze H₂O₂ and generate ·OH to oxide the ciprofloxacin.

A comparison table would be interesting to show earlier reported work in “heterogeneous Fenton-like oxidation of Ciprofloxacin” area.

-- We have added a comparison table in page 11.

Author Response File: Author Response.pdf

Reviewer 2 Report

Catalysts

Manuscript number: 685311

Title: A novel reduced graphene-attapulgite (rGO-ATP) supported Fe2O3 catalyst for heterogeneous Fenton like oxidation of Ciprofloxacin: degradation mechanism and pathway

 

This work deals with the synthesis and application of a reduced graphene-attapulgite (rGO-ATP) supported Fe2O3 catalyst for the removal of ciprofloxacin by heterogeneous Fenton oxidation. Although the topic could be interesting, there is a clear lack of relevance in this work. On the one hand, the introduction is too general and the choice of the proposed catalyst is not well supported. On the other hand, the characterization of the catalyst is not complete. Important information such as the actual loading of iron and the specific surface area is not given. Furthermore, and more important, the results obtained are not reliable as the technique used for the quantification of ciprofloxacin does not allow to separate the different compounds present in solution and the intermediates interfere in the quantification. Other important aspects such as following global parameters like TOC and H2O2 along reaction, as well as a kinetic study and the stability of the catalyst have not even been considered. For all these reasons, I cannot recommend this paper for publication. Apart from these comments, please consider the following ones:

Major:

The abstract requires an overhaul. The information contained is too general. The optimum results obtained together with the operating conditions tested should be clearly shown. In the same line, the main properties of the catalyst should be also included. Advanced oxidation processes are not current methods for the removal of emerging pollutants in water. They constitute a main field of research but, in practice, adsorption is the most widely applied process for the removal of these kind of compounds (see for instance the Swiss scenario). Some aspects deserve a deeper explanation in the introduction. For instance, what is the removal yield of ciprofloxacin in current wastewater treatment plants? What is its residual concentration in the aquatic environment? What is the relevance of the catalyst developed in the current work? How the combination of reduced graphene and attapulgite as support could improve the activity of Fe2O3? What are the previous results reported for the Fenton oxidation of ciprofloxacin? The characterization techniques used in the current work do not allow to clearly confirm the presence of iron in the catalyst. What was the actual loading of iron? TXRF or ICP techniques could help. On the other hand, the specific surface area of the solid should be also given. The initial concentration of ciprofloxacin tested is extremely high compared to the expected concentrations in WWTP effluents. What is the theoretical stoichiometric amount of H2O2 to achieve the complete mineralization of ciprofloxacin? The doses tested in the current work should be compared to such concentration. Figure 4 is misleading. It is stated that ciprofloxacin concentration is depicted but, in fact, the lines represent the absorbance spectrum. Definitely, the quantification of ciprofloxacin by spectrophotometry is not reliable. Different hydroxylated intermediates will be formed during the first stages of reaction and they interfere the quantification. Section 2.3 does not contain any relevant information. Once the degradation of ciprofloxacin has been achieved, the three-dimensional fluorescence image shows a significant intensity. According to the authors, it can be explained by the presence of other intermediates, which, in fact, allows to corroborate that this technique should not be used for quantification of ciprofloxacin. The separation of the different species present in solution (e.g. by HPLC) is required. Important global parameters such as TOC and H2O2 should have been followed along reaction to check the effectiveness of the process. The stability of the catalyst has not been considered. At least, iron leaching should be followed. The results obtained in the current work should be critically compared in the context of the literature. To what extent does the proposed catalyst improve previous conventional catalysts? A table summarizing previous results and the ones achieved in this work would be helpful.

Minor:

According to the IUPAC, hydroxyl radicals should be written as HO· instead of ·OH.

 

Author Response

Dear reviewer

Thanks for your useful comments and suggestions on the language and the structure of our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point:

The abstract requires an overhaul. The information contained is too general. The optimum results obtained together with the operating conditions tested should be clearly shown. In the same line, the main properties of the catalyst should be also included.

-- We have rewritten the abstract.

Advanced oxidation processes are not current methods for the removal of emerging pollutants in water. They constitute a main field of research but, in practice, adsorption is the most widely applied process for the removal of these kind of compounds (see for instance the Swiss scenario). Some aspects deserve a deeper explanation in the introduction. For instance, what is the removal yield of ciprofloxacin in current wastewater treatment plants? What is its residual concentration in the aquatic environment? What is the relevance of the catalyst developed in the current work? How the combination of reduced graphene and attapulgite as support could improve the activity of Fe₂O₃? What are the previous results reported for the Fenton oxidation of ciprofloxacin?

-- We have given more details in the Introduction.

The characterization techniques used in the current work do not allow to clearly confirm the presence of iron in the catalyst.

-- The characterization techniques used in the current work clearly confirmed the presence of iron in the catalyst. Please see to FT-IR analysis, XRD analysis and XPS analysis of Fe₂O₃/RGO-ATP catalyst in pages 4-6.

What was the actual loading of iron? TXRF or ICP techniques could help.

-- Considering time limit, we did not do TXRF or ICP test for the catalyst, while the loading of iron has been studied in our other researches.

On the other hand, the specific surface area of the solid should be also given.

-- We have provided specific surface area, total pore volume of the as-synthesized materials in page 4 lines 142-146.

The initial concentration of ciprofloxacin tested is extremely high compared to the expected concentrations in WWTP effluents. What is the theoretical stoichiometric amount of H₂O₂ to achieve the complete mineralization of ciprofloxacin? The doses tested in the current work should be compared to such concentration.

-- In this study the initial concentration of ciprofloxacin was prepared to simulate a kind of high-concentration and hard-to-degrade organic wastewater in order to test the catalysis performance of the as-synthesized. And the catalytic system in this study cannot complete mineralize ciprofloxacin; just can obtain the intermediate products of degradation.

Figure 4 is misleading. It is stated that ciprofloxacin concentration is depicted but, in fact, the lines represent the absorbance spectrum. Definitely, the quantification of ciprofloxacin by spectrophotometry is not reliable.

--The results of Figure 4 were tested by a three-dimensional fluorescence spectrometer, not by a spectrophotometer.

Section 2.3 does not contain any relevant information. Once the degradation of ciprofloxacin has been achieved, the three-dimensional fluorescence image shows a significant intensity. According to the authors, it can be explained by the presence of other intermediates, which, in fact, allows to corroborate that this technique should not be used for quantification of ciprofloxacin. The separation of the different species present in solution (e.g. by HPLC) is required.

-- The three-dimensional fluorescence can be used in ciprofloxacin concentration determination according to the references, and the three-dimensional fluorescence image can be used for qualitative analysis of ciprofloxacin.

Important global parameters such as TOC and H₂O₂ should have been followed along reaction to check the effectiveness of the process. The stability of the catalyst has not been considered. At least, iron leaching should be followed.

-- Considering time limit, we did not test the TOC of the treated water. The stability of the catalyst has been considered and data have been added in page 8 line 261. Iron leaching experiments have been done but we cannot test any iron leaching in the solution so we don’t have any data about it.

The results obtained in the current work should be critically compared in the context of the literature. To what extent does the proposed catalyst improve previous conventional catalysts? A table summarizing previous results and the ones achieved in this work would be helpful.

-- We have added a comparison table in page 11.

 

Reviewer 3 Report

 

This work explains the Fe2O3/rGO based catalyst (heterogeneous) to analyze the degradation of ciprofloxacin for Fenton reaction.
This manuscript is an example of good technical work but below average introduction writing, and presentation. Work is suitable for MDPI Catalysts. I would recommend the publication of this paper after the following issues are addressed adequately.

 

Defect density is an important feature for graphene materials (G, GO, rGO) but author has not done any Raman experiments or even mentioned it to analyze how Raman defects affect the degradation mechanism. Author is at least advised to mention the details which could be along the lines of-

"Graphene morphological defects affect the overall performance of the material.

Carbon, 98 (2016), pp. 491-495, https://doi.org/10.1016/j.carbon.2015.10.083

Nanoscale, 2017,9, 5872-5878, 10.1039/C7NR00308K"

 

 

The introduction is good for
Ciprofloxacin and ATP and their derivatives. However, the introduction significantly lacks a discussion on graphene family and relevant properties that have been harnessed in the presented research work by the author.
e.g. previous work on GNP (graphene) morphology, crystallinity, decomposition, surface properties, and their encapsulation with other materials which is of utmost importance for this paper. The perspective readers have to look at a lot of other references to understand the introduction and discussion part, and authors could have done better work at a literature review of GNP as well.

Author should have shown characterization results to comment on the stated parameters. At the least, a few sentences should be added to the introduction. A few suggestions that cover all of the above properties and applications.

-J. Am. Chem. Soc.2011, 133,9, 2816-2819. (DOI:10.1021/ja109793s)

-ACS Appl. Mater. Interfaces 2016, 8, 45, 30899-30907

-Mater. Horiz., 2019,6, 796-801. (doi: 10.1039/C8MH01507D)

 

Line 102- '.....as shown in red in the figure ." where is this red in the figure?

Author needs to re-read the manuscript and correct the small errors.

fig 1- correct the figure labels a, b, c.
Also, correct (enlarge font) the figure scale-bars, they are not visible in the present format.

Figure 4- Mention the spectroscopic characterization technique name (is it three-dimensional fluorescence spectrometer ?) used for this experiment in the caption.

 

Line 288- graphite and/or rGO flake or sheet size is not mentioned. G is available for 1um size and also 200um which will change the entire mechanism of degradation reaction. Author must mention the specific details of G and resulting rGO for their experiments.

 

Author Response

Dear reviewer

Thanks for your useful comments and suggestions on the language and the structure of our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point:

 

Defect density is an important feature for graphene materials (G, GO, rGO) but author has not done any Raman experiments or even mentioned it to analyze how Raman defects affect the degradation mechanism. Author is at least advised to mention the details which could be along the lines of-"Graphene morphological defects affect the overall performance of the material.

Carbon, 98 (2016), pp. 491-495, https://doi.org/10.1016/j.carbon.2015.10.083

Nanoscale, 2017,9, 5872-5878, 10.1039/C7NR00308K"

-- We have done Raman experiments and discussed them about defect density.

The introduction is good for Ciprofloxacin and ATP and their derivatives. However, the introduction significantly lacks a discussion on graphene family and relevant properties that have been harnessed in the presented research work by the author.

e.g. previous work on GNP (graphene) morphology, crystallinity, decomposition, surface properties, and their encapsulation with other materials which is of utmost importance for this paper. The perspective readers have to look at a lot of other references to understand the introduction and discussion part, and authors could have done better work at a literature review of GNP as well.

--GNP is not the key point in this study, if we give more details of GNP, the introduction part would be too excessive.

Author should have shown characterization results to comment on the stated parameters. At the least, a few sentences should be added to the introduction. A few suggestions that cover all of the above properties and applications.

-J. Am. Chem. Soc.2011, 133,9, 2816-2819. (DOI:10.1021/ja109793s)

-ACS Appl. Mater. Interfaces 2016, 8, 45, 30899-30907

-Mater. Horiz., 2019,6, 796-801. (doi: 10.1039/C8MH01507D)

--We have added some characterization results to comment on the stated parameters.

Line 102- '.....as shown in red in the figure ." where is this red in the figure?Author needs to re-read the manuscript and correct the small errors.

--We have corrected the errors.

Fig 1- correct the figure labels a, b, c.Also, correct (enlarge font) the figure scale-bars, they are not visible in the present format.

--We have corrected the figure labels and figure scale.

Figure 4- Mention the spectroscopic characterization technique name (is it three-dimensional fluorescence spectrometer ?) used for this experiment in the caption.

-- We have the spectroscopic characterization technique name in the caption.

Line 288- graphite and/or rGO flake or sheet size is not mentioned. G is available for 1um size and also 200um which will change the entire mechanism of degradation reaction. Author must mention the specific details of G and resulting rGO for their experiments.

-- We synthesized a kind of catalyst with 3-D structure, not flake or sheet structure. The pore size, which is given in the manuscript, is more important than sheet size.

 

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

In this manuscript, the authors reported that reduced graphene oxide-attapulgite (RGO-ATP) supported Fe₂O₃ catalyst and then systematically characterized to study for heterogeneous Fenton-like oxidation of Ciprofloxacin. Although the approach is interesting, the present paper contains several weak points, originating from writing style. More careful English correction is needed for the whole manuscript. Many errors are found (wrong numbered Figure, unnamed table titles, etc.). In my opinion, although the manuscript contains a moderate scientfic level of experimental results, it should be rewritten. Authors are suggested to rewrite the whole manuscript in order to correct typos and poorly constructed sentences.   

Author Response

Dear reviewer

Thanks for your useful comments and suggestions on the language and the structure of our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point:

First:

1.In this manuscript, the authors reported that reduced graphene oxide-attapulgite (RGO-ATP) supported Fe₂O₃ catalyst and then systematically characterized to study for heterogeneous Fenton-like oxidation of Ciprofloxacin. Although the approach is interesting, the present paper contains several weak points, originating from writing style. More careful English correction is needed for the whole manuscript. Many errors are found (wrong numbered Figure, unnamed table titles, etc.). In my opinion, although the manuscript contains a moderate scientfic level of experimental results, it should be rewritten. Authors are suggested to rewrite the whole manuscript in order to correct typos and poorly constructed sentences. .

-- We reorganized the content to make it easier to understand. At the same time, the article was sent to a specialized agency for language editing, and errors such as untitled table titles and incorrect numbering in the article were corrected.

Author Response File: Author Response.pdf

Reviewer 3 Report

The author has made efforts to resolve the issues and this manuscript is in good shape now.

Fig. 1 scalebar is very small to analyze the sem pictures

Font size is too small to read easily for axis labels of most of the plots in this manuscript.

Author Response

Dear reviewer

Thanks for your useful comments and suggestions on the language and the structure of our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point:

 

1.The author has made efforts to resolve the issues and this manuscript is in good shape now.

Fig. 1 scalebar is very small to analyze the sem pictures

Font size is too small to read easily for axis labels of most of the plots in this manuscript.

-- We have adjusted all the graphics in the article.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

The authors have improved all issued from the original submission and can be accepted.  However, the author should carefully check some copied texts and sentences in revised manuscript. I suggest the authors modify/rephrase those sentences.